Replace ivygen leaf cards with custom leaves
Most often you don't want to do that: think about the amount of polygons that would be. (You could kinda get away with it if you work with Ivygen at a small scale. Good luck with doing it with a tree in the Sapling add-on...)

The smart way as long as the camera is far enough to get away with it is to alpha map the leaves. (I believe they are UV unwrapped by default, I could be mistaken.) This reduces the poly but gives you a greater amount of detail.

If you want an example of this technique out in the wild, have a look at this piece from blenderartists: http://blenderartists.org/forum/showthread.php?382855-Fairytale-Village

If the camera is too close for a pure alpha map, you may want to use dupliobject (and specifically duplifaces) to give it some depth. These approaches can be mixed: the exact outline of the leaf can be done using alpha maps to save on polygons. For duplifaces, see here: You could use dupliobjects. See here: http://wiki.blender.org/index.php/Doc:2.4/Manual/Modeling/Objects/Duplication/DupliFaces